Field of the Invention
This invention relates generally to heterocyclic compounds for organic electroluminescent devices. The invention further relates to heterocyclic compounds containing dibenzothiophene or dibenzofuran molecular structure in the core skeleton and phosphorescent organic electroluminescent devices using the same.
Description of Related Art
Recently, organic electroluminescent devices (OLED) are considered to be emissive display technology competitive to other display technologies such as liquid crystal displays (LCDs) and light emitting diodes (LEDs). OLED devices are commercially attractive because they offer the cost-advantageous fabrication of high density pixeled displays exhibiting brilliant luminance, high efficiency, low driving voltage, with true color and longer operational stability.
A typical OLED comprises at least one emitting layer sandwiched between an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton”, which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes through a photoemissive mechanism. To improve the charge transport capabilities and also the luminous efficiency of such devices, additional layers around the emitting layer, such as an electron transport layer and/or a hole transport layer, or an electron blocking and/or hole blocking layer(s) have been incorporated. Doping the host material with another material (guest) has been well demonstrated in literature to enhance the device performance and to tune the chromaticity. Several OLED materials and device configurations are described in U.S. Pat. Nos. 4,769,292, 5,844,363, and 5,707,745, which are incorporated herein by reference in their entirety.
More recently, OLEDs having emissive materials that emit light from triplet states (phosphorescence) have been demonstrated in literature, Nature, 1998, No. 395, p. 151 and Appl. Phys. Lett., 1999, No. 3, p. 4, and U.S. Pat. No. 7,279,704, which are incorporated herein by reference in their entirety.
Selection of a host material in phosphorescent OLEDs is difficult especially since the non-emissive triplet excited state of the host material must be higher than that of the guest phosphor (dopant). In addition, a host material must have good charge transport properties for an efficient OLED. JP2001-313178 disclosed CBP (4,4′-bis(N-carbazolyl)-1,1′-biphenyl) as the host material, which is characterized by having a good hole transport property but poor electron transporting. Hence the use of CBP as a host material for tris(2-phenylpyridine) iridium (hereinafter referred to as Ir(ppy)3), a green phosphorescent emitter, disturbs balanced injection of electrical charges, causing excess holes to flow towards the electron transport layer, thereby decreasing the luminous efficiency. Moreover, due to its low molecular weight, it tends to crystallize and thus is not suitable for OLED devices.
One of the means to solve the above problem is to introduce a hole blocking layer between the emitting layer and the electron transport layer as described in JP2002-305083. This hole blocking layer accumulates holes efficiently in the emitting layer and contributes to increase the probability of recombination of holes and electrons and thus enhances the luminous efficiency. Currently, the hole-blocking materials in general use include 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (hereinafter referred to as BCP) and phenylphenolato-bis(2-methyl-8-quinolinator-N1,08)aluminum (hereinafter referred to as BAlq). However, BCP tends to crystallize even at room temperature and lacks reliability as a hole blocking material and the life of the device is extremely short; whereas BAlq has insufficient hole blocking ability.
For a high luminous and efficient OLEDs, a host material must have non-emissive high triplet energy and a balanced electrical charge (hole/electron) injection/transport characteristics. Moreover, the host material should also possess good electrochemical stability, high thermal resistance and excellent thin film stability. However, compound capable of satisfying all the said properties from practical considerations have not been known till date.
Attempts have been made to introduce molecular moiety that has an excellent hole transport property as represented by a carbazole or triarylamine and another moiety that has an excellent electron transport property as represented by pyrimidine or triazine into one and the same molecular skeleton, as phosphorescent host materials, as disclosed in the patent documents WO2003-78451, WO2005-76668, US2006-51616, JP2008-280330, WO2008-123189 and JP 2009-21336.
Dibenzothiophenes (DBT) and dibenzofuran (DBF) are some of the heterocyclic moieties having a high triplet energy and a high mobility, which does not shown to have intense absorptions in the visible region. Its co-planarity is quite favorable for intermolecular interaction. Patent documents U.S. Pat. No. 8,007,927, KR20110085784, U.S. Pat. No. 8,409,729 and US20140151649 disclose the use of benzodithiophenes or dibenzofurans used in the light emitting devices.
However, it still needs a desired OLED having characteristics in terms of high luminance, operational stability and reduced driving voltage.